Overview:

Commercial solid rocket motors use various grain geometries that differ from what we employ on the MIT Rocket Team. This page is meant to describe the grain geometries of different industry rockets and their effects on flight performance in order to learn and adapt to our own designs.

Space Shuttle Booster:

The forward segment of the motor employs an 11-point star shape along with a domed closure at the front. The middle segments use a straight-walled cylinder shape (BATES). The aft segment is mostly BATES, but tapers outward to surround the booster's submerged nozzle. As seen in the thrust curve, this combination was designed to create a boost to get off the launch pad before decreasing thrust to handle maximum dynamic pressure. The domed closure and submerged nozzle were likely designed for a structural reason rather than pure grain shape, with the grain serving to fill up the otherwise empty space (as opposed to MIT Rocket Team's normal cylindrical tube).

Ariane 5 P-230 Booster:

The forward segment of the booster has a 15-point star grain geometry along with a domed forward closure. The forward segment also includes faster burning fuel than the rest of the motor to alter the thrust curve. The center segments are straight-walled (BATES) grain. In the aft segment, the BATES tapers outward to enclose a submerged nozzle. The thrust of the motor is as follows: it increases with the expanding area of the BATES and star shapes, it decreases substantially when the forward segment burns out, and finally it increases with the BATES until it also burns out.

Vega P80 Motor:

The forward section of the motor has a BATES shape and a forward domed closure. The middle section also has a cylindrical BATES grain geometry. At the aft end, the BATES opens up into a 12-point star shape before surrounding a submerged nozzle. These geometries have the thrust first increase with the expanding BATES/star shapes, decrease as the star loses area and becomes more circular, and finally increase with the expanding areas before burning out.

Titan IV UA-1207:

The forward booster segment includes an 8-point finocyl grain geometry along with a forward domed closure. The center segments are cylindrical in shape, but all also slightly taper outward in the aft direction. The aft segment is cylindrical (BATES) with a domed shape. As seen in the Thrust vs. Time graph, adding restrictors to the segments can significantly change the thrust curve over time. In the standard configuration, however, it has a regressive curve, where the grain area continuously decreases over time before burnout.

Conclusions:

  • Complex/large designs seen in industry require segmented motors
  • Many industry grains use 3D shapes (such as truncated cones)
  • Industry motors use domed ends filled with fuel (probably due to structural reasons)
  • Many industry grains surround a submerged nozzle (likely not too useful for Rocket Team)
  • Some motors have segments with different burn rates to further alter thrust curves
  • Most industry motors are relatively simple (mainly cylindrical with a little bit of finocyl grain)
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